The use of inexpensive plastic food packaging containers and sheets has become popular for dispensing and serving various food products. Generally, these food containers and sheets are typically made of a olefin polymer, such as polypropylene, or polyethylene, or a styrenic polymer such as polystyrene, including high impact polystyrene.
One drawback of the above materials for making plastic food packaging containers and sheets is the lack of sufficient flexural modulus coupled with good impact and good heat resistance (i.e., microwaveable). A material that lacks sufficient flexural modulus, polyethylene or polypropylene for instance, will require a much greater thickness to make the container or sheet functional from the rigidity point of view. The increase in material thickness, however, increases the cost of manufacturing. If a material has a low impact property, polystyrene, for instance, its resulting container or sheet is more brittle and, therefore, more likely to break during use than a container or sheet made with a material having a higher impact property. A sheet or container made with a material having a low heat resistance will result in the loss of modulus or stiffness at higher temperatures, making the container or sheet not functional.
By adding a mineral filler to the polyolefin, rigidity can be increased. One filler widely used is talc. There are tradeoffs, however, in using talc as a filler. For example, the impact property of a talc-filled polyolefin container decreases as the wt. % of talc in the container or sheet increases. As mentioned above, a container or sheet with a low impact is brittle and can easily fracture or shatter upon dropping. Talc loading of greater than about 50 wt. % usually results in a composite which is too brittle to be used in most applications. For some applications, however, it is desirable to use more than 50 wt. % filler because that decreases the cost of the container or sheet and makes the container or sheet more rigid.
Accordingly, a need exists for a container or sheet, such as a polyolefin container or sheet, that has a desirable flexural modulus while also having a desirable impact property and heat resistance.